Technical Abstract: Predicting crop residue amount and distribution was needed to meet objectives of the Wind Erosion Prediction System (WEPS). Residues are maintained in three pools: standing, flat, and buried. Different environments in each pool result in different decomposition rates. The model predicts that standing residues weaken and fall to the soil surface as they decompose. Climatic indices provide indicators of decomposition environments. the minimum of daily moisture and temperature indices is accumulated as decomposition days. Decomposition of below-ground residues is based on soil temperature and water content, simulated in the hydrology submodel. A climatic index based on daily precipitation and air temperature is used to predict above-ground biomass decomposition and the rate that standing stems fall. When daily precipitation exceeds 4 mm, the moisture index is optimum. The wetting effect persists over time, decreasing by 60% daily until re-wetting. The temperature coefficient is optimum at 32 C. Decomposition is predicted as a first-order decay process, with decomposition days used as the time scale and crop-specific decomposition rate coefficients base on residue quality. When wind is potentially erosive, the erosion submodel uses percent soil cover, based on biomass in the standing and flat residue pools, Stem Area Index, based on number of standing stems per unit land area.